A recently developed type of secondary or rechargeable electrical conversion device comprises: [1] an anodic reaction zone containing a molten alkali metal anode-reactant, e.g., sodium, in electrical contact with an external circuit; [2] a cathodic reaction zone containing [a] a cathodic reactant comprising sulfur or a mixture of sulfur and molten polysulfide, which is electrochemically reversible reactive with the anodic reactant; [b] a solid electrolyte comprising a cation-permeable barrier to mass liquid transfer interposed between and in contact with the anodic and the cathodic reaction zones; and [c] electrode devices within the cathodic reaction zone for transporting electrons to and from the vicinity of the cation-permeable barrier. As used herein the term "reactant" is intended to mean both reactants and reaction products.
During the discharge cycle of such a device, molten alkali metal atoms such as sodium surrender an electron to an external circuit and the resulting cation passes through the solid electrolyte barrier and into the liquid electrolyte to unite with polysulfide ions. The polysulfide ions are formed by charge transfer on the electrode by reaction of the cathodic reactant with the electrons conducted through the electrode from the external circuit. Because the ionic conductivity of the liquid electrolyte is less than the electronic conductivity of the electrode material, it is desirable during discharge that both electrons and sulfur be applied to and distributed along the surface of the electrode in the vicinity of the cation-permeable solid electrolyte.
During the charge cycle of such a device when a negative potential larger than the open circuit cell voltage is applied to the anode the opposite process occurs. Thus, electrons are removed from the alkali metal polysulfide by charge transfer at the surface of the electrode and are conducted through the electrode material to the external circuit, and the alkali metal cation is conducted through the liquid electrolyte and solid electrolyte to the anode where it accepts an electron from the external circuit. Because of the aforementioned relative conductivities of the ionic and electronic phases, this charging process occurs preferentially in the vicinity of the solid electrolyte and leaves behind molten elemental sulfur.
It is the principal object of this invention to provide a structure for containing an alkali metal battery in which electrical insulation is provided between the anodic and cathodic reaction zones and a secure seal is formed so that the reactants are not lost from the battery.